1. Field of the Invention
The present invention relates to polymers which are useful in oil recovery as fracturing fluids. More particularly, the invention provides a fracturing fluid composition which includes as a thickening agent a carboxyalkyl derivatized polygalactomannan and a zirconium crosslinking agent wherein the resulting fluid demonstrates excellent stability over a period of time at high temperatures.
2. Technology Description
It is well known that production in petroleum, natural gas and geothermal wells can be greatly enhanced by hydraulic fracturing techniques. These techniques are known in the art and generally comprise introducing an aqueous solution of a water-soluble polymer (e.g. Guar Gum) in which "proppants" (e.g. coarse sand or sintered bauxite or synthetic ceramic materials) are suspended through the well bore under extremely high pressures into the rock structure in which the petroleum, gas or steam is entrained. Minute fissures in the rock are thereby created and held open by the suspended particles after the liquid has drained off. The petroleum, gas or steam can then flow through the porous zone into the well.
Polysaccharides e.g. guar and guar derivatives are the most commonly used water-soluble polymers for hydraulic fracturing. Aqueous solutions of guar and guar derivatives develop increased viscosity upon the addition of various metal ions. Viscoelastic gels are formed by the chemical linking or cross-linking of two or more polymer chains. The result is a more ordered network structure which increases the effective molecular weight and thereby, the viscosity. The stability of these high viscosity cross-linked gels is dependent on many factors including pH and temperature. The viscosity stability of water-soluble polymer solutions as a function of time and temperature, is crucial for successful applications in the oil field area.
Thermal stability is a major factor in selecting a water-soluble polymer for wells having high bottom-hole temperatures. It is well known that crosslinked fracturing fluids degrade with time as a function of temperature and shear, resulting in a loss of viscosity and proppant carrying ability in a short time at temperatures of 250.degree. F. and above.
The observed loss of viscosity as a function of time, temperature and shear, is the result of degradation by several pathways, for example chemical, biological and mechanical. Biological degradation can be minimized by the proper choice of biocide. Mechanical degradation is the result of applying a critical stress to the gel, resulting in chain scission. Its effect can be minimized by the use of properly engineered surface equipment, etc.
There are two chemical pathways of importance. One is hydrolysis of the glycosidic linkage, resulting in scission of the polysaccharide chain. The other is oxidative/reductive depolymerization. Acid catalyzed hydrolysis of the glycosidic bond is well documented. The rate of degradation by glycosidic bond hydrolysis is dependent on reaction time, system pH and temperature. Oxidative/reductive depolymerizations involve the oxidation of the polysaccharide by a radical pathway in the presence of oxygen. Transition metal ions, (e.g. iron) can promote these processes. This thermal degradation of the gels can be minimized by the addition of oxygen scavengers such as sodium thiosulfite, methanol, thiourea and sodium thiosulfate and by avoiding extremely high or low pH conditions.
In practice, carboxymethylhydroxypropyl guar has been the polygalactomannan of choice for use in fracturing fluids. This material has shown an excellent ability to effectively crosslink and establish a desirable viscosity profile. However, despite its widely successful use, improvements can be made in discovering other thickening agents which provide excellent long term stability at high temperatures without being expensive to manufacture.
Examples of art discussing fracturing fluids which include polygalactomannan thickening agents, preferably carboxymethylhydroxypropyl guar include the following: U.S. Pat. No. 5,305,832; U.S. Pat. No. 4,477,360; U.S. Pat. No. 4,488,975; U.S. Pat. No. 4,579,670; U.S. Pat. No. 4,692,254; U.S. Pat. No. 4,702,848; U.S. Pat. No. 4,801,389; U.S. Pat. No. 5,103,913; U.S. Pat. No. 4,460,751; U.S. Pat. No. 4,579,670; and U.S. Pat. No. 5,271,466. While several of these patents suggest that carboxyalkyl guar polygalactomannans may be used as thickening agents, they fail to recognize that under specific conditions, carboxyalkyl guars can be used which demonstrate excellent viscosity retention while utilizing a minimal amount of crosslinking agent.
Despite the above teachings, there still exists a need in the art for novel fracturing fluid formulations which demonstrate excellent viscosity retention at high temperatures and which utilize a minimal amount of chemical materials.